To create useful electrical current from electromagnetic radiation, photovoltaic (PV) cells must absorb incident radiation such that an electron is promoted from the valence band to the conduction band (leaving a hole in the valence band), and must be able to separate the electron and hole and deliver these charge carriers to their respective electrodes before they recombine.
Many different strategies based on diverse materials have been employed, with varying degrees of success, to realize these basic behaviors with commercially satisfactory efficiency. Representative devices include crystalline inorganic solar cells (e.g., silicon, germanium, GaAs), nanocrystalline dye-sensitized solar cells, semiconductor-polymer solar cells, nanoparticle solar cells, and more recently, composite solar cells that incorporate and combine the aforementioned components from other strategies.
1. Inorganic Photovoltaics
Silicon is by far the most commonly used material for fabricating inorganic photovoltaics. These cells rely on the ability of silicon to absorb light and, consequently, to generate an excited electron-hole pair that is then separated at a p-n junction. The electric field set up by the p-n junction facilitates this separation because of the way electrons and holes move through materials: electrons move to lower energy levels while holes move to higher energy levels.
Creation of p-n junctions generally involves high-temperature processing in inert atmospheres to form very pure, crystalline silicon wafers, which are inflexible and expensive. Because silicon is an indirect semiconductor, a relatively thick layer is typically needed to achieve a good level of absorption, which increases material costs further. Efficiencies for the most pure (and expensive) silicon photovoltaics are on the order of 20%; efficiencies for the cheaper amorphous silicon cells are approximately 5-10%.
Today's commercial PV systems can convert from 5% to 15% of sunlight energy into electricity. These systems are highly reliable and generally last 20 years or longer. The possibility of fabricating solar cells by less expensive, lower-temperature techniques is very attractive. Accordingly, nanocrystalline dye-sensitized solar cells (DSSCs), semiconductor-polymer solar cells and nanoparticle solar cells have enjoyed widespread interest.
2. Polymer Photovoltaics
Semiconducting polymers can be used to make organic photovoltaics. The properties of these polymers can be tuned by functionalization of the constituent monomers. As such, a wide range of polymers with suitable bandgaps, absorption characteristics and physical properties is available. In order to achieve separation of the electron-hole pair, organic photovoltaics rely on donor-acceptor heterojunctions. In polymers, the excited-state electron and hole are bound together, and travel together, as a quasi-particle called an exciton. They remain together until they encounter a heterojunction, which separates them. Unfortunately, excitons are very short-lived and can only travel about 10 nm before recombining. Hence, any photon absorbed more than this diffusion length away from a heterojunction will be wasted. Charge mobilities for polymers are typically low (0.5−0.1 cm2 V−1 s−1) compared to silicon, which is much higher (1500 cm2 V−1 s−1). Current state-of-the-art polymer photovoltaic cells have efficiencies of 1-2%. Although such efficiencies are low, these materials hold promise for low-cost, flexible solar cells.
3. Nanoparticle Photovoltaics
Inorganic nanoparticles (or nanocrystals) have been used to prepare colloidal, thin-film PV cells that show some of the advantages of polymer photovoltaics while maintaining many of the advantages of inorganic photovoltaics. For example, such cells can contain a bi-layer structure comprising a layer of donor and a layer of acceptor nanoparticles, wherein the two layers exhibit little intermixing, and both contribute to the measured photocurrent. The strong photoconductive effect exhibited by these devices suggests that these materials have a large number of trapped carriers and are better described by a donor-acceptor molecular model than by a p-n band model. Increased bandgap energy compared to that of the bulk semiconductors minimizes the number of carriers available, and spatial separation of the donor and acceptor particles in different phases traps the excitons so that they must split at the donor-acceptor heterojunction. There is no band-bending, so splitting of the exciton is more difficult.
It should be stressed that simply blending the donor and acceptor nanoparticles together will not create a film that produces a photovoltage. The lack of selectivity at the electrode towards one particle or another means that the electrodes can make contact with both the donor and acceptor species. These species may take the form of nanorods rather than nanospheres because nanorods with high aspect ratios help to disperse the carriers. Quick transfer of the exciton along the length of the nanorods improves the chance of splitting the exciton at the donor-acceptor heterojunction.
Solution processing of, for example, CdSe rods can achieve a size distribution of 5% in diameter and 10% in length with an aspect ratio of 20 and a length of 100 nm. The substantial control available through solution processing allows for optimization of the cell by variation of nanorod length and bandgap energy.
4. Polymer-Nanocrystal Composite Photovoltaics
The combination of nanomaterials and polymer films has been shown to give good power conversion efficiencies while affording low-temperature solution processes for fabrication. In one approach, nanomaterials are used to conduct charges while the polymer is used as the absorbing material, or alternatively, the nanomaterial serves as a chromophore, i.e., the light absorber, and the semiconductor polymer is employed as a hole conductor. In the former case, a wide-bandgap semiconductor (e.g., TiO2) receives the excited electron from the conduction band of the chromophoric polymer semiconductor; and in the latter case, light-absorbing semiconductor nanocrystals absorb photons and transfer the resulting negative charge to the transparent primary electrode, while the semiconducting polymer transfers the holes to the counter electrode. In both types of cell, a heterojunction between the nanocrystal and the polymer separates the exciton created in the nanocrystal or polymer. The electron is transferred to the conduction band of the nanocrystal and the hole stays in the valence band of the polymer, or the electron stays in the conduction band of the nanocrystal, and the hole is transferred to the valence band of the polymer.
4.1 Wide-Bandgap Nanocrystal/Light-Absorbing Polymer
The active layer in a polymer-nanocrystal cell has two components: a light absorber and a nanoparticulate electron carrier. Typically, the light absorber is a p-type polymeric conductor, e.g., poly(phenylene vinylene) or poly(3-hexylthiophene), and the nanoparticulate electron carrier is a wide-bandgap semiconductor such as ZnO or TiO2. In this configuration, the polymer serves to absorb light, to transfer electrons to the electron acceptor/carrier, and to carry holes to the primary electrode. The electron acceptor accepts electrons and transfers the electrons to the metal back contact.
The morphology of the phase separation is crucial. For example, a bi-layer structure in which each layer has only one component results in a cell with poor performance. The reason is that the lifetime of the excited state of the light-absorbing polymer is generally shorter than the transfer rate of the exciton to the interface, and, consequently, the majority of the excitons formed in the bulk of the polymer never reach the interface separating electrons and holes, resulting in loss of photocurrent. Morphologies in which a bulk heterojunction is formed tend to show greater efficiencies. If the absorber and electron acceptor are in intimate contact throughout the entire active layer, the shorter exciton path length will result in increased electron transfer and higher efficiencies. The best efficiencies obtained from cells of this configuration are around 2%.
This technology shows promise, but there are obstacles to overcome. One problem is incomplete absorption of the incident radiation. The polymer—which absorbs light very strongly and is referred to as a polymeric dye—has a large extinction coefficient (>100,000 M−1 cm−1), but due to low exciton migration rates, the films must generally be thinner than 100 nm, which contributes significantly to incomplete absorption. This effect can be combated by means of an interdigitated array structure of donor and acceptor species.
4.2 Wide-Bandgap Nanocrystal/Light-Absorbing Nanocrystals/Hole Transfer Polymer
A problem associated with the light-absorbing polymer strategy is underutilization of available solar energy due to the narrow absorption bandwidth of typical polymers. Approximately 40% of the light (from about 600 nm out into the near IR) can be wasted. An alternative configuration is to utilize nanocrystals as light absorbers and electron carriers, and employ the polymer as a light absorber and a hole carrier. CdSe nanorod and tetrapod/polymer systems have demonstrated power-conversion efficiencies of up to 1.7%. These systems have the advantage that the absorption of the nanocrystal can be tuned via the size of the nanocrystal, and systems that absorb essentially all of the incoming radiation can therefore be fabricated.
Unfortunately, it is difficult to disperse inorganic nanocrystals into a solution of monomers. The two phases tend to agglomerate and minimize the electrical contact essential to form the heterojunction which enables charge separation. Dispersion of nanocrystals in polymer phases is an area of great interest.
Typically, the strategy employed for dispersing the nanocrystals is to functionalize the nanocrystal with a capping agent that has an organic tail, which enhances solubility in the solvent in which the polymerization is carried out. Capping agents for this purpose typically have a head-group with a strong affinity for the nanocrystal; amine, carboxylate, phosphine, thiol, phosphine oxide and phosphonic acid, for example, all bind strongly. The organic tail of the capping agent should be compatible with solvents in which the polymer is soluble. Long hydrocarbon chains typically provide high solubility but are non-conducting; accordingly, it is necessary to balance optimum solubility against conductivity.
The most popular polymers used for composite studies are PDFC, P3Ht and MEH-PPV (where PDFC refers to -{poly[9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(9-ethyl-3,6-carbazole)]}-, P3Ht refers to poly(3-hexylthiophene), and MEH-PPV refers to poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene)). Each of these polymers has sites for functionalization, allowing the manipulation of the valence/conduction band energies to achieve optimal conditions for charge transfer to and from the nanocrystals. It has been suggested that the capping agent may also serve as the organic acceptor phase; for example, P3HT functionalized with phosphonic acid groups has been shown to isolate CdSe nanocrystals.
5. Dye-Sensitized Solar Cells
DSSCs incorporate a substrate which has been coated with a transparent conducting oxide (which serves as the primary electrode). The counter electrode may also be coated with a transparent conducting oxide, but may also be a non-corrosive metal, such as titanium coated with a very thin layer of platinum. A porous layer of a wide-bandgap semiconductor (such as TiO2) is deposited on the conductive surface of the primary electrode. This porous layer is then coated with a dye having a strong absorption in the visible region of the spectrum. To be optimally effective, the dye concentration should be limited to a monolayer of dye molecules. Because of this, a huge surface area is necessary to accommodate enough dye to absorb all of the incoming light. Therefore, nanocrystals (e.g., TiO2) are used to make the highly porous films. Electrolyte containing a redox couple (typically I−/I3−) is absorbed into the titania layer. To complete the cell, the substrate bearing the primary electrode and the sensitized titania layer is brought into face-to-face contact with the counter electrode.
Typical dyes are inorganic-ruthenium-based, although organic dyes are receiving increased interest. The dye absorbs visible light, and the excited state injects an electron into the TiO2 conduction band. Before back electron transfer can occur, the oxidized dye is reduced by a redox active species in solution (typically I−/I3−), regenerating the dye. The oxidized redox active species diffuses to the counter electrode, where it is reduced, finishing the cycle and completing the circuit. Work can be done by passing the injected electron through an external load before allowing it to reduce the oxidized redox active species at the counter electrode.
Inexpensive DSSC devices, which exhibit up to 10% energy conversion efficiency, can be fabricated. There are many issues to be addressed with this technology to improve performance and stability, including replacing the best performing liquid electrolytes with solid-state or higher-boiling electrolytes; improving spectral overlap; using a redox mediator with a lower redox potential; and lowering recombination losses due to poor electron conduction through the nanoparticle TiO2 layer.
6. Hybrid Cells
Hybrid cells combine dye-sensitized titania, coated and sintered onto a transparent semiconducting oxide, with a p-type polymer that carries electrons to the oxidized dye. Since just one polymer replaces the multi-component electrolyte, these cells can be fabricated conveniently and reproducibly. Ruthenium dye-sensitized, nanorod-based DSSCs tend to exhibit low efficiency, however, because the lower surface area does not accommodate enough dye to absorb all of the incident light. The most efficient dyes found so far only have extinction coefficients on the order of ˜20,000 M−1 cm−1, and therefore a large surface area is needed to bind enough dye to get maximal absorbance.